Spatial tracking systems are used for various procedures such as patient surface feature extraction, surgical navigation, or treatment delivery guidance for patient positioning. One critical performance criterion of such systems is their spatial accuracy—i.e., how accurately the tracking systems identify and track the physical location of objects with respect to one or more known reference coordinate systems.
The tracking system is typically composed of an optical tracker anchored to a wall or ceiling of a room, or alternatively placed on a tripod, whose position is calibrated relative to a fixed reference coordinate system. The tracker senses signals (e.g., infrared, magnetic, radio, etc.) that emanate from active markers that are attached to the surface of, or embedded within, the object, or alternatively from passive markers that reflect signals emanating from the tracker itself. Using triangulation techniques, the position and orientation (6 DOF) of the markers, and by proxy the object to which the markers are attached, can be calculated with respect to the optical tracker, and through a transformation, with respect to a fixed reference coordinate system. In other embodiments, the tracker can track surface elements of an object directly without the use of markers, such as in the case of a mounted camera or laser scanning system which images the object's surface elements (e.g. points on a patient's skin) and, through a transformation, calculates the position of these surface elements with respect to a fixed reference coordinate system.
However, the accuracy of such a system depends on maintaining a constant relationship between the fixed coordinate system and the tracking system. If, for example, the tracking system is accidentally knocked out of its calibrated position, or if the tracker drifts over time, inaccuracies may be introduced such that the transformation between the tracker's coordinate system and the fixed coordinate system is no longer accurate. As a result, coordinates assigned to individual features of the object (e.g., marker position, surface, pixels, lesions, etc.) will be misaligned with respect to the fixed coordinate system. Such misalignments can lead to, for example, inaccurate surgical operations or incorrect delivery of radiation treatments, resulting in potentially harmful outcomes.